1. Field of the Invention
The present invention relates to an ignition control apparatus for an internal combustion engine. More particularly, it relates to an ignition control apparatus provided with a plurality of ignition timing data maps which are selected in accordance with changes in the operating condition of the engine.
2. Description of the Related Art
An internal combustion engine as applied to an automobile and provided with an electric control system, such as a micro-computer system adapted for effective control of the engine, is well known in the art.
Such a system is used to control various operating parameters of the internal combustion engine, for example, ignition timing in a spark ignition type engine, in which the control apparatus controls the optimum value of the ignition timing in accordance with engine operating parameters, such as engine speed and engine load.
Known in the prior art is an electric control system for an internal combustion engine wherein the air-fuel ratio of the air-fuel mixture to be introduced into the engine combustion chambers is controlled to a theoretical value. This system utilizes ignition timing data maps, and an ignition timing is calculated by a map in accordance with the engine load and the engine speed.
Recently, due to improvements made to sensor devices and control devices, a system has been proposed wherein the air-fuel mixture is controlled in the area leaner than that providing the theoretical air-fuel ratio, to save fuel consumption. Such a control of the air-fuel mixture is referred to hereinafter as "lean A/F control". In such lean A/F control, the combustion is controlled in the lean area under a stable running condition of the engine. When the engine is required to provide a high output, the combustion is also controlled in an area corresponding to the theoretical air-fuel ratio and sometimes in the area which is richer than the area corresponding to the theoretical air-fuel ratio. In this system, the air-fuel ratio is therefore widely varied in accordance with the conditions of the engine, which makes it difficult to obtain an optimum ignition timing calculated by a single map. The ignition timing calculated by a single map can, of course, be arithmetically corrected in accordance with the operating condition of the engine. The correction is, however, insufficient to attain optimum control of the ignition timing in every area of the engine, and thus a deterioration occurs in the drivability, knocking is caused, and the fuel consumption efficiency becomes degraded.
This difficulty is worsened if the engine is provided with a swirl control system for attaining a stable combustion with lean A/F control, since selection of the ignition timing in this case becomes very complex.
This difficulty is more or less encountered also when the air-fuel ratio is controlled to the stoichiometric value.